1. Field of the Invention
This invention relates to a process for preparing a catalyst for removing nitrogen oxides and more particularly, to a process for preparing a catalyst which does not oxidize sulfur dioxide on removal of nitrogen oxides.
2. Description of the Related Art
It has been already known that a titanium oxide sintered product is used as a carrier or catalyst. Since the surface area, crystal form, mechanical strehgth and heat resistance, which give an important influence on the performance of the carrier or catalyst, vary depending on the manner of preparation, the absence or presence, type and amount of additive, various processes have been heretofore provided.
For instance, it is known that metatitanic acid or orthotitanic acid, which are readily obtained by thermal hydrolysis of titanates, are converted by sintering into anatase-type titanium oxide. This type of titanium oxide is generally, though not essentially, considered to be a favorable crystal form for use as a carrier or catalyst and has been used as a starting material for a carrier on which a conventional catalyst for removing nitrogen oxides is to be supported.
In a method in which silica is added to the thus obtained titanium hydroxide or titanium oxide and sintered, however, it is difficult to obtain a mixture of a uniform composition. Especially, where silica is added to titanium hydroxide which is in the form of a gel, it is not possible to uniformly disperse silica to titanium hydroxide. Thus, there cannot be obtain a carrier or catalyst of a high performance.
According to a method in which an additive, such as silica, is added to orthotitanic acid, a mixture of a uniform composition is also difficult to obtain. Since orthotitanic acid is generally in the form of a gel, it is difficult to uniformly disperse an additive to orthotitanic acid, so that there cannot be obtain a carrier or catalyst of a high performance.